This invention relates to a method of producing binary shape-memory alloy films by sputtering process techniques. In particular it relates to a method of producing shape-memory nickel titanium alloy films using hot pressed metal targets in the sputtering process.
Various metallic materials capable of exhibiting shape-memory characteristics are well known in the art. These shape-memory capabilities occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. In particular, it was discovered that alloys of nickel and titanium exhibited these remarkable properties of being able to undergo energetic crystalline phase changes at ambient temperatures, thus giving them a shape-memory. These alloys, if plastically deformed while cool, will revert, exerting considerable force, to their original, undeformed shape when warmed. These energetic phase transformation properties render articles made from these alloys highly useful in a variety of applications. An article made of an alloy having a shape memory can be deformed at a low temperature from its original configuration, but the article xe2x80x9cremembersxe2x80x9d its original shape, and returns to that shape when heated.
For example, in nickel-titanium alloys possessing shape-memory characteristics, the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is often referred to as a thermoelastic martensitic transformation. The reversible transformation of the NiTi alloy between the austenite to the martensite phases occurs over two different temperature ranges which are characteristic of the specific alloy. As the alloy cools, it reaches a temperature (MS) at which the martensite phase starts to form, and finishes the transformation at a still lower temperature (Mf). Upon reheating, it reaches a temperature (AS) at which austenite begins to reform and then a temperature (Af) at which the change back to austenite is complete. In the martensitic state, the alloy can be easily deformed. When sufficient heat is applied to the deformed alloy, it reverts back to the austenitic state, and returns to its original configuration.
Shape-memory materials previously have been produced in bulk form, in the shape of wires, rods, and plates, for utilities such as pipe couplings, electrical connectors, switches, and actuators, and the like. Actuators previously have been developed, incorporating shape-memory alloys or materials, which operate on the principal of deforming the shape-memory alloy while it is below its phase transformation temperature range and then heating it to above its transformation temperature range to recover all or part of the deformation, and, in the process of doing so, create moments of one or more mechanical elements. These actuators utilize one or more shape-memory elements produced in bulk form, and, therefore are limited in size and usefulness.
The unique properties of shape-memory alloys further have been adapted to applications such as micro-actuators by means of thin film technology. Micro-actuators are desirable for such utilities as opening and closing valves, activating switches, and generally providing motion for micro-mechanical devices. It is reported that the advantageous performance of micro-actuators is attributed to the fact that the shape-memory effect of the stress and strain can produce substantial work per unit of volume. For example, the work output of nickel titanium shape-memory alloy is of the order of 1 joule per gram per cycle. A shape-memory film micro-actuator measuring one square millimeter and ten microns thick is estimated to exert about 64 microjoules of work per cycle.
The most well known and most readily available shape-memory alloy is an alloy of nickel and titanium. With a temperature change of as little as about 10xc2x0 C., this alloy can exert a force of as much as 415 MPa when applied against a resistance to changing its shape from its deformation state. However, there exists a challenge to develop low cost, reliable processes for producing microns-thick, thin films of these shape-memory alloys.
Now an improved process for fabricating shape-memory alloy thin films using sputtering techniques has been developed.
According to the present invention, there is provided a method for producing a thin film deposit of a shape-memory alloy film wherein a hot pressed metal target is employed in the sputtering process.
While the use of hot pressed targets in a sputtering process for producing thin films is a previously practiced technique, the use of such target materials in producing shape-memory alloy films has not been recognized as a viable approach. The inherent porosity of a pressed powder composition dictated against the use of hot pressed targets for thin film sputtering of shape-memory alloys, because of the potential for trapped oxygen. Oxygen contamination was found to have adverse effects on shape-memory properties of NiTi alloys, such as by suppressing martensite transformation and degrading the mechanical properties of the alloys.
Phase transition temperatures and shape-memory properties of NiTi shape-memory alloys are very sensitive to the composition. It was found that 1 at % of redundant nickel could decrease Ms temperature by 100xc2x0 C., and the redundant titanium could substantially degrade the mechanical properties of the material. To ensure consistent transition temperatures and good shape-memory effect (xe2x80x9cSMExe2x80x9d) properties, composition control is the key. In NiTi thin film fabrication processing by sputtering, the first source of composition variation of thin film is from target. Therefore, target composition has to be closely controlled.
To minimize oxygen contamination, typically, sputtering targets for shape-memory alloy films are fabricated using alloy process techniques involving numerous steps including melting, remelting, solidification, and even long-time homogenization treatment. Because very high temperature has to be used, these procedures often result in the preferential loss of one or more elements to the others, and the compositional control becomes very difficult. Along with high cost and long processing time, the difficulty in compositional control makes this target-making process very impractical, especially when making large size targets.
According to the present process, a homogenous target for sputtering deposition is accomplished by using hot pressing powder metallurgy techniques. In addition to quicker and easier fabrication, and significantly facilitated compositional control, it unexpectedly has been found that binary shape-memory alloy thin films produced by sputtering using such hot pressed targets exhibit excellent mechanical properties and shape-memory effect, as well as widely ranged phase transition temperatures.